Prior art on thermally-trimmable resistors addresses trimming of such resistors housed in thermally-isolated microstructures. The microstructures offer substantial thermal isolation, allowing the microstructure to be raised to a high temperature using a minimal amount of power, while the temperature of the surrounding chip remains at a low temperature. Typical thermal isolation for cantilevers or membranes used in thermally-trimmable resistors is tens of degrees Kelvin of temperature rise per mW dissipated in the microstructure. For example if a microstructure has thermal isolation 50K/mW, then 20 mW dissipated in a heater-resistor in that microstructure, would raise the local on-microstructure temperature by 1000° C., which would result in thermal trimming of a functional resistor also housed in that same microstructure. Note that the heater-resistor may or may not be the same resistor as the functional thermally-trimmable resistor, and may or may not be made of the same materials as the functional thermally-trimmable resistor.
In many cases of practical manufacture of thermally-trimmable resistors, it may be advantageous to use more than one microstructure to house a single functional resistor having a specific target resistance value. For example, one may want to use one or more cantilever-shaped microstructure(s) of a particular standard size, to create thermally-trimmable functional resistors having different resistance values. For example, in such a case the cantilever size may be restricted due to limitations in the manufacturing technology (e.g. stress in the films, time needed for the microstructure release etch, mechanical robustness of the microstructure as a function of its size, and/or a fixed range of sheet resistance of the resistor film material). Thus, one may want to electrically connect the functional resistance traces from more than one cantilever, in series or parallel, and treat the resulting multiplicity as one device, thermally-trimming them all simultaneously with common trimming signals applied to the heater-resistors of each cantilever.
In some typical cases of thermal trimming, the heater-resistors are also thermally-trimmable, and in some cases are subject to failure (open-circuit), when subjected to high power and resulting high temperatures. Note that in cases where the heater-resistor and functional resistor are not the same, the temperature within the functional resistor is always somewhat less than the temperature within the heater-resistor, because the heater resistor is the source of the heat. Device failure can be brought about by excessive temperature and typically the trimming is limited by failure in the heater-resistor. For example in a single cantilever-shaped microstructure, where a separate heater-resistor and functional resistor are both polysilicon thin films, the trim-down range of the functional-resistor may be greater than 40%, and is limited beyond this point by open-circuiting of the heater-resistor.
Typically, the “trim range” or “trim-down range” refers to the specified maximum induced resistance change downwards (decreasing the resistance from its as-manufactured value) at the point where trimming ceases, usually as a result of failure of the heater-resistor (or aggregate heater, in the case where more than one microstructure is used). In many cases, due to the connectivity of the heater-resistors in an aggregate circuit, when one of the heaters fails (becomes open-circuited), it may disable (for a variety of reasons) any further heating (signaling the end of trimming). Barring severe manufacturing defects affecting a heater in a specific microstructure, the first heater-resistor to fail is typically in the “hottest” microstructure. Under normal operation, the hottest microstructure should also contain the functional resistance portion trimmed furthest down, meaning that all other microstructures have not reached their full trim-down potential. In effect, the hottest microstructure limits the overall adjustment range of the aggregate thermally-trimmable resistor.
In the case of a multi-microstructure or multi-cantilever resistor, if all of the microstructures/cantilevers were identically-shaped, with identical thermal isolation, and if all of the heater-resistors had identical resistance, then ideally all of the functional resistance traces could experience the same temperatures over time, and trim identically in unison. However, in practice, even if all of the materials and shapes and resistances were initially identical (initially, before any trim-heating signals are applied), if the micro-structures are positioned near each other in a silicon chip, the heat from trim-heating signals will be shared, to a non-zero extent, causing spatial non-uniformities in temperature, and causing unequal temperatures experienced by the (otherwise-identical) microstructures.
Typically, deep trim-downs require the highest trimming temperatures, and one may not raise the heater-temperatures indefinitely—eventually, when higher and higher temperatures are reached, the heater-resistor is likely to eventually fail, giving an open-circuit. Therefore, certain microstructures are likely to be closer to failure, and their corresponding functional resistor traces are likely to be trimmed down further, than their neighboring microstructures.
With such non-uniformities in temperature, each microstructure may experience a different trimming temperature, and thus different trimming behavior.